There is a type of vehicle back door structure that includes an upper window, lower window, and a rear spoiler located between the two windows. A prior art example of such a vehicle back door structure will now be described. FIG. 3 is a cross-sectional view of a back door showing a portion where a rear spoiler is located. As shown in 3, a back door 100 includes an upper window 102, a lower window 104, rear spoiler 106 located between the two windows 102 and 104, and a back door frame 108 that supports the two windows 102 and 104 and the rear spoiler 106. The back door 100 is of a type that swings upward and opens and closes a back door opening of a vehicle (not shown).
The back door frame 108 includes a window frame that defines upper and lower window openings 111 and 112. As shown FIG. 3, the window frame includes a partition 115 that partitions the two window openings 111 and 112. The upper window 102 is attached to the back door frame 108 so as to seal the upper window opening 111. The lower window 104 is attached to the back door frame 108 so as to seal the lower window opening 112. The rear spoiler 106, which is formed from a resin, is formed as a hollow structure having a low profile in the vertical direction of the vehicle. The rear spoiler 106 is fastened to the back door frame 108 by bolts and nuts 116. The rear spoiler 106 projects toward the rear of the vehicle.
Patent document 1 describes a vehicle back door structure including a resin panel formed by resin-molding and integrating a window and a rear spoiler located above the window.